There is an ongoing effort in the electronics industry to achieve increasing functionality in a decreased case size. This desire has placed an ongoing burden on component manufacturers to achieve ever more functionality with minimal size. Towards this goal the instant application is primarily focused on improved capacitors, and particularly electrolytic capacitors.
In general, a capacitor comprises an anode and cathode separated by a dielectric. The anode is typically manufactured by pressing and sintering the powder. An anode lead extends from the anode. The dielectric is formed on the anode and the cathode is formed on the dielectric. Anode and cathode terminations are added to facilitate attachment of the capacitor to an electrical circuit.
The desire to improve capacitors has led to the evaluation, and use, of tantalum and niobium powders with ever increasing capacitance as a function of volume. The powders, referred to in the art as high CV powders, are desired because of their reduced power consumption. Unfortunately, with increased formation voltage these powders exhibit a phenomenon referred to as CV roll down which limits the application of high CV powder in high working voltage capacitors. The phenomenon, which is known in both Ta and Nb capacitors, is believed to be caused by the growth of anodic oxide films through the necks between powder particles. These oxide films clog the pores between the particles in sintered anodes thereby resulting in reduced anode surface area. The reduced anode surface area, as a function of volume, is believed to contribute to CV roll down.
The dielectric is typically an amorphous oxide matrix of the underlying anode metal. It is difficult to increase the formation voltage of high CV powders due to the precipitation of a crystalline oxide phase in the amorphous matrix of the anodic oxide film. Crystalline inclusions in the amorphous oxide are believed to inhibit the formation of a thick dielectric film on the anode surface which provokes high and unstable d.c. leakage. These precipitates are typically associated with impurities, particularly bulk oxygen, in Ta(Nb) anodes. This has led to efforts to reduce the oxygen concentration in Ta(Nb) anodes.
U.S. Pat. Nos. 5,825,611 and 6,410,083 and 6,554,884 are representative of attempts to address the crystalline oxide problem by treating the Ta or Nb anodes with nitrogen to purge oxides while limiting nitride precipitates. U.S. Pat. No. 4,537,641 describes the reduction of bulk oxygen content in Ta(Nb) anodes by adding reducing agent to the anodes. Mg, for example, was added to the anode and the anodes were heated above the melting point of the reducing agent but below the temperature conventionally used for sintering. During the heating, the reducing agent reacts with oxygen in Ta(Nb), creating magnesium oxides on the anode surface. These magnesium oxides are then leached in aqueous solutions, for instance diluted sulfuric acid and hydrogen peroxide, when anodes are exposed to air after sintering.
An alternative process, based on the combination of sintering and deoxidizing, is disclosed in U.S. Pat. No. 6,447,570. According to the process, the Ta(Nb) powder is pressed into a pellet and Mg is added to the pellets. The pellets and Mg are placed in crucibles in a vacuum oven, or covered with inert gas and heat treated to generate Mg vapor. The Mg vapor reacts with oxygen thereby effectively deoxidizing the Ta(Nb). The deoxidized Ta(Nb) is then sintered in vacuum or inert gas. Due to the decrease in oxygen concentration the sintering can be done at lower temperatures. Sintering at lower temperatures results in improved morphology of the sintered anodes and a stronger bond between the powder particles and lead wire. After sintering, the pellets are cooled and simultaneously treated with nitrogen to reduce Ta(Nb) affinity for oxygen. After cooling the Mg oxide is leached from the anode surface in diluted water solution, preferably of H2SO4 and H2O2, leaving bulk Ta(Nb) particles which are practically free of oxygen. The improved morphology, low oxygen, and strong powder-to-wire bonding in Ta(Nb) anodes results in high volumetric efficiency and low d.c. leakage in the finished Ta(Nb) capacitors. The disadvantage of the technology is the complexity and inefficiency of the equipment required. During deoxidizing, Mg vapor spreads through the reaction chamber and condenses on all cold parts including electrical insulation on the heaters. During consequent sintering in vacuum, or in inert gas, Mg shunts can cause shortage of the power and control circuits. This method therefore requires an extensive, and frequent, cleaning procedure to remove residual Mg which typically must be performed after each run of the furnace. These problems prohibit large scale production.
The art is still lacking a method of providing a capacitor which achieves the potential offered by high CV powders. Either the adhesion between the lead wire and pressed powder is insufficient or the manufacturing process is inoperative on a large scale. Those of skill in the art are still seeking a method of utilizing high CV Ta(Nb) powders and full realization of the potential suggested thereby. Such a method is provided by the present invention.